Defining keys_for

The ConfusionMatrix class requires a keys_forProc that returns a
symbol. In this method, you specify what constitutes a true positive
(:tp), true negative (:tn), false positive (:fp), and false
negative (:fn). For example, in spam classification, you can construct
the following table to write the keys_for method:

Once you have your keys_for method implemented, pass it into the
ConfusionMatrix with method(:keys_for), or if it's a class-method,
MyClass.method(:keys_for). (You can also implement the method as a
lambda.)